128 ALTERNATING CURRENTS 



at once evident from the fact that the polyphase alternating currents 

 flowing in the armature windings give rise to a rotating field, and 

 if we imagine the magnet wheel to be rotating at the same speed 

 that of synchronism and to be suitably placed relatively to the field 

 due to the armature currents, a driving torque will be exerted on it. 

 As in the case of a single-phase machine, a polyphase synchronous 

 motor is only capable of running at one particular speed that of 

 synchronism. For otherwise we should get an irregular succession 

 of driving and retarding impulses, whose mean algebraic value 

 is zero. 



Another way of regarding such a polyphase motor is to consider 

 each armature phase as acting independently, and giving rise to a 

 fluctuating driving torque, such as we get in a single-phase machine. 

 The fluctuating torques due to all the phases become fused into a 

 single steady driving torque. 



On account of the fact that alternators, whether single- or poly- 

 phase, when used as motors are only capable of running at one 

 particular speed that of synchronism they are termed synchronous 

 motors. 



69. Stability of Synchronous Motor 



We have assumed the alternating current to be so adjusted as 

 to give rise to the necessary value of the driving torque required 

 to overcome all the resistances to the motion. We shall now show 

 that when a motor is so running, with a definite p.d. across its 

 terminals, it is in a condition of stability i.e. any tendency on the 

 part of the motor to run faster or slower, due to a decrease or increase 

 of load, is automatically checked by a suitable change in the magnitude 

 and phase of the current. 



It is obvious that in order to provide the necessary driving torque, 

 the succession of impulses contributed by the current in any one 

 phase need not necessarily be of the same sign ; we may, for example, 

 have each large driving impulse succeeded by a smaller retarding 

 impulse, so that there is, on the whole, a preponderance of driving 

 impulses, and a resultant mean driving torque. Such a succession 

 of alternate impulses will occur if the reversal of current does not 

 take place at the precise moment when an armature conductor is 

 passing from a field of one polarity into a field of opposite polarity ; 

 and corresponding to each wave of current there will be four im- 

 pulses two large driving ones and two small retarding ones. 



Let us suppose that the p.d. across the armature of the synchronous 

 motor and the exciting current supplied to its field are maintained 

 constant. Let V = p.d., E = open-circuit e.m.f. corresponding to 



